Solid state membranes formed from oxygen ion-conducting materials continue to show promise in a variety of commercial processes including the separating of oxygen from oxygen-containing gaseous mixtures. Representative solid-state membranes are those formed from multicomponent metallic oxides which are typically operated at high temperatures (e.g. 700.degree. C. or more) wherein the solid-state membranes conduct both oxygen ions and electrons. When a difference in oxygen partial pressure exists on opposite sides of the mixed conducting metallic oxide membrane and operating conditions are properly controlled, oxygen is separated from the oxygen-containing gaseous mixture as oxygen ions migrate to the low oxygen partial pressure side of the solid-state membrane while an electron flux occurs in the opposite direction of oxygen ion migration in order to conserve charge, producing pure oxygen on the permeate side of the membrane.
A plurality of solid state membranes may be joined together to form a membrane module wherein channels are incorporated into each respective membrane unit in order to facilitate introducing the oxygen-containing gaseous mixture to be separated into the module and recovering the oxygen product from the module. As shall be further described in this Specification, Applicants have discovered that the dense mixed conducting oxide layer spanning the supporting channels is susceptible to mechanical failure when a pressure differential is applied across the solid-state membrane units of the membrane module. Moreover, the channeled layers of the membrane units making up the module are somewhat difficult to fabricate.
Gas separation modules and fuel cells of the prior art are typically operated under conditions such that a near zero pressure differential exists across the membrane cells wherein problems associated with pneumatic integrity are minimized and minor leaks are tolerated to a limited extent between the cells. Moreover, the effective active surface area of the dense mixed conducting separating layer of the individual membranes is restricted by the channeled layers which typically support the dense mixed conducting separating layer of the membranes. These modules must be manifolded in a configuration so that oxygen can exit through the collection channels within each membrane unit.
Fuel cell modules formed from a plurality of cells are well known in the art. Representative fuel cells are disclosed in U.S. Pat. No. 4,490,445 which teaches a solid oxide electrochemical energy converter comprising alternating layers of solid oxide electrolyte plates and electrical conductor plates. Each electrolyte plate includes a coating of a porous oxidizer electrode on a first surface of the electrolyte and a coating of a porous fuel electrode on a second surface of the electrolyte. Each conductor plate includes grooved networks formed by ridges which define gas passages on both surfaces of the conductor plate, such ridges being in electrical contact with the electrode coatings on next adjacent electrolytes. Each conductor plate also possesses a means for tapping electricity from or introducing electricity into the converter. The conductor plates also possess circumferential ridges arranged along the edges of the conductor plate to define gas seals, the ridges being in contact with surface coatings on next adjacent electrolyte plates which surface coatings possess the same composition as that of the electrode coatings.
U.S. Pat. No. 4,791,079 discloses two-layer conducting catalytic ceramic membranes which are suitable for use in a variety of hydrocarbon conversion reactions. The two-layer membrane possesses one layer formed of an impervious mixed ion and electronic conducting ceramic such as yttria stabilized zirconia which is doped with sufficient CeO.sub.2 or titanium dioxide to impart electron conducting characteristics to the ceramic. A second layer associated with mixed conducting impervious ceramic is a porous ion conducting layer containing a selective hydrocarbon oxidation catalyst.
A solid electrolyte oxygen pump formed from a plurality of solid-state membranes is presented in U.S. Pat. No. 4,877,506. The oxygen pump possesses electrodes which are shaped to form a plurality of linear, parallel channels on facing surfaces of the electrolyte. The air feed is introduced into the channels formed of the air electrode. Oxygen formed during operation of the device is removed by passage through the electrolyte via channels formed of the oxygen electrode or anode. A monolithic array is formed by situating an interconnecting material between adjacent cells to form a stack of cells.
U.S. Pat. No. 5,034,023 discloses ceramic honeycomb structures which are capable of separating oxygen from an oxygen-containing gaseous mixture. The channeled honeycombs are formed from a solid electrolyte having at least some of the honeycomb channels sealed at one of its faces. The oxygen-containing gas is introduced into a first set of channels at one face of the honeycomb, a first voltage is applied to the interior walls of the channels and a second voltage is applied to the interior walls of the second set of remaining channels thereby creating an electrical potential across the ceramic material separating adjacent channels of the two sets. The electrical potential drives oxygen ions through the channel walls releasing molecular oxygen into the second set of channels which can be collected.
U.S. Pat. No. 5,045,169 discloses an electrochemical device capable of generating oxygen from air upon the application of an electrical current, where a plurality of adjacent electrochemical cells are electrically connected in series, each cell containing an inner, porous oxygen electrode; a dense, solid oxide electrolyte capable of transporting oxygen ions partly disposed on top of the inner electrode and partly disposed between inner electrodes of adjacent cells; an outer porous air electrode disposed on top of the electrolyte; and separate, dense, electronically conductive segments of interconnection material disposed between adjacent cells, the interconnection electrically and physically connecting the outer air electrode from one cell to the inner oxygen electrode from an adjacent cell, the device having gas impermeable, dense, contacting segments of electrolyte and interconnection material between inner electrode of adjacent cells.
U.S. Pat. No. 5,240,480 discloses representative solid-state membranes for separating oxygen from oxygen-containing gaseous mixtures. These membranes comprise a multicomponent metallic oxide porous layer having an average pore radius of less than about 10 micrometers and a multicomponent metallic oxide dense layer having no connected through porosity wherein the porous layers and dense layers are contiguous and such layers conduct electrons and oxygen ions at operating temperatures.
U.S. Pat. No. 5,356,728 and European Patent Application WO 94/24065 disclose cross-flow electrochemical reactor cells formed from multicomponent metallic oxides of the perovskite structure which demonstrate electron conductivity and oxygen ion conductivity at elevated temperatures. Such cells are useful in carrying out partial oxidation reactions of organic compounds to form added-value products and separating oxygen from oxygen-containing gaseous mixtures.
The cross-flow reactor cells of U.S. Pat. No. 5,356,728 comprise either a hollow ceramic blade positioned across a gas stream flow containing one or more channels for flow of gas streams or a stack of crossed hollow ceramic blades containing one or more channels for flow of gas streams. Each channel has at least one channel wall disposed between a channel and a portion of an outer surface of the ceramic blade or a common wall with adjacent blades in a stack comprising a gas impervious multicomponent metallic oxide, typically of a perovskite structure, which exhibits electron conductivity and oxygen ion conductivity at elevated temperatures. Thus, the channels are contiguous to the outer surface of the ceramic blade which is formed from the multicomponent metallic oxide.
Industry is searching for solid-state membrane modules which are suitable for conducting a wide variety of processes and reactions wherein the modules would exhibit improved pneumatic and structural integrity. Moreover, such modules would desirably be readily fabricated and manifolded and would be capable of withstanding the pressure differential necessary in practicing air separation processes and desirable in practicing partial oxidation processes. Such modules would desirably not possess structural elements such as channels which are in contact with the dense mixed conducting oxide layer because such channels limit the effective active surface area of the dense mixed conducting oxide layer of each membrane unit. Such channels render the membrane units of prior art solid state membrane modules susceptible to mechanical failure when a pressure differential is applied across the membrane units of the module.